The objective of this project is to investigate and characterize an unusual variant of the AKR murine leukemia line derived from a spontaneous tumor arising in a 14 month old mouse which had been thymectomized at one month of age. This system was selected as a model for a rarely occurring natural variant of AKR leukemia in which the host animal has an atrophic thymus rather than the usual thymoma. Two passage lines of AKR tumors were established and continued by intravenous injection of 1 x 10 to the 5th power spleen cells into young AKR hosts. Donor cells were initially derived from tumors arising in 14 month old animals, one of which had been totally thymectomized at one month of age and one of which had been partially thymectomized, had a thymoma in the thymic remnant and was used as a control. The passage has been presently carried nine passage generations during which cells were studied for various characteristics including Theta antigen (by surface immunofluorescence and cytotoxicity), surface immunoglobulin (SIg), in vitro synthesis of immunoglobulin, and mitogen responsiveness. Preliminary attempts at study of aggregate receptors, Ia antigens and passages of cell-free (viral) centrifuges from the two lines are in progress or planned.